Means for detecting ground insulation failure for rotary electric machines

ABSTRACT

Electrical potentials are induced in the rotating shaft of many rotary electrical machines because of stray flux that exists in the machines. Unless provided with adequate ground insulation to interrupt the circuit, currents can flow through the machine bearings and other parts to ground and thereby expose the machine to severe damage. The device of this invention measures the voltage across the shaft and the bearing to indicate if there is a circuit for excessive current leakage from the bearing to ground.

United States Patent 1191 Burrus Sept. 9, 1975 [54] MEANS FOR DETECTING GROUND 3,176,286 3 1965 Dschen 340 269 x INSULATION FAILURE FOR ROTARY 3,596,135 7/1971 Stenger, Jr. et al 324/54 X ELECTRIC MACHINES Joe H. Burrus, Brookfield, Wis.

Allis-Chalmers Corporation, Milwaukee, Wis.

Filed: Oct. 10, 1974 Appl. No.: 513,876

Inventor:

Assignee:

US. Cl 317/262 R; 324/54; 340/269; 317/13 R 1m. (:1. G01R 31/12; G08B 21/00 Field 61 Search 317/13 R, 262 R; 324/54; 340 269 References Cited UNITED STATES PATENTS Kresge 324 54 x Primary ExaminerL. T. Hix Attorney, Agent, or FirmJohn P. Hines ABSTRACT 2 Claims, 1 Drawing Figure MEANS FOR DETECTING GROUND INSULATION FAILURE FOR ROTARY ELECTRIC MACHINES This invention pertains in general to rotary electrical machines and more particularly to a device for detecting a failure of insulation provided to interrupt the circuit between the machine bearing and electrical ground.

In many large rotating electrical machines such as, for instance, steam turbine-generators, the machine bearing is mounted in a pedestal separated from the main generator housing or in a bracket as part of the generator end cover. Because of stray flux existing in the generator, a voltage is induced in the generator shaft as it rotates. Although the oil film which exists between the shaft and the bearing and between other parts of the machine does offer an insulating effect to this voltage, the low insulating value of an oil film does not offer adequate protection. If the bearing housing were grounded, the current flowing therethrough would damage the bearing, requiring premature replacement thereof. Since replacement of bearings on this type of machine can result in considerable expense to the owner, it is extremely important to properly insulate the bearing housing from electrical ground.

Not only is a proper ground insulation necessary, but procedures must be employed to insure that the insulation does not deteriorate. In most instances, a periodic reading is taken across the bearing housing and ground to insure that the insulation is satisfactory. Voltage sensing devices are not permanently connected across the bearing housing and ground because these devices may short and nullify the insulation thereby causing damage to the bearings.

It is the intention and general object of this invention to provide a means for detecting deterioration of the bearing ground insulation on rotary electrical machines without providing a path for current flow through the detector to ground.

Another object of the subject invention is to provide a device of the hereinbefore described type wherein a voltage sensing means is provided across the oil film that separates the rotary electric machine shaft and the bearing housing.

A more specific object of the subject invention is to provide a device of the hereinbefore described type wherein alarm indicating means are connected to the voltage sensing means to indicate when the ground insulation is deteriorating and when the insulating oil film between the shaft and the bearing is insufficient.

These and other objects of the subject invention will become more fully apparent as the following description is read in light of the attached drawing which shows a schematic view of a rotary electrical machine utilizing the invention disclosed herein.

Referring to the attached drawing, the invention shown herein for purposes of illustration is a steam turbine-generator with the turbine indicated at 5, the generator at 6, and the outboard bearing housing at 7. Due to stray flux existing in the generator 6, a voltage is induced in the shaft 8. The discharge circuit for this voltage is shaft 8 through inboard bearings or grounding brush 9, through bearing insulation 11 to bearing housing 7, through oil film 10 to shaft 8. The oil film 10 and bearing insulation 1 1 act as electrical capacitors in the circuit, limiting the flow of current to a few micro amperes. If the outboard bearing housing is not insulated from ground, there would be sufficient current flow therethrough to seriously damage the bearings and cause premature'replacement thereof.

Since the cost of the bearings, the cost of replacement and the downtime of the generator can be extremely costly to the generator owner, it is important to insure that the ground insulation 11 does not deteriorate to the extent that increased current will flow and damage the bearings. It is possible to provide a voltage or current sensing means across the bearing housing 7 and ground which would warn the operator when the ground insulation was deteriorating to a dangerous level. However, since any instrument is subject to fail ure, the sensing device could, if defective, provide a direct short to ground through the bearing and thereby do more damage to the bearing than would a deteriorating ground insulation. For this reason sensing devices have not been extensively utilized at this location.

The device of this invention is an extremely uncomplicated means of determining when the ground insulation begins to deteriorate. Furthermore, this device does not provide a potential direct short between the bearing housing and ground.

The oil film and the bearing housing to ground insulation act as two electrical capacitors in series. A voltage can be measured across the oil film between the shaft and the bearing pads 12 and also across the ground insulation 11 between the bearing housing 7 and electrical ground. The amount of the voltage from shaft to ground varies, depending on the character of the particular machine involved. Generally, the sum of these two voltages may be as little as 4 volts or as high as 30 volts. As an example in a particular machine, there may be a total of 10 volts between the shaft 8 and the electrical ground. This voltage is divided between the oil film and the ground insulation. As an example, there may be 2 volts across the oil film and 8 volts across the ground insulation 11. Since the total voltage from the shaft to ground remains substantially constant, any change in the voltage across the ground insulation, due to high resistance shorts, would result in a change in the voltage across the oil film. That is, if the voltage in the above example across the ground insulation changed from 8 to 6, there would be an increase in the voltage across the oil film. If the voltage across the insulation went to zero, the voltage across the oil film would rise to 10. By sensing the change in the voltage across the oil film, it is possible to immediately determine the voltage change across the ground insulation.

With this in mind, applicant provides a voltage sensing means across the oil film between the shaft 8 and the bearing housing 7. This voltage sensing means may be an electronic voltage sensing device which emits a signal upon change of voltage from a preset amount. As an example, two voltage sensing means 13 and 14 may be provided across the oil film 10. Each voltage sensing means is adjustable to a preset voltage within the range to be measured. Each voltage sensing means is also provided with a warning device such as a light 15 and 16.

The warning device of the subject invention would be installed and operated as follows. It would first be necessary to insure that the machine was operating satisfactorily and that the ground insulation 1 1 and oil film 10 were adequate, and also that shaft contact brush 17 is functioning. For ease of illustration, assume that there is a lO-volt potential from the shaft 8 to ground across the bearing housing 7. Assume further that the voltage across the oil film is 2 volts. It would then be known that the voltage across the ground insulation 11 is 8 volts. The voltage sensing device 14 would then be set at, for instance, 1 volt, and the sensing device 13 would be set at, for instance, 3 volts.

Under normal operating conditions, the voltage across the oil film 10 would be approximately 2 volts and neither warning light would flash However, if the voltage increased to 4 volts, this would indicate a decrease in voltage across the ground insulation and an increase in current. Since the voltage sensing device 13 was set to emit a signal if the voltage increased beyond 3 volts, the warning light 16 would flash, indicating to the operator that the ground insulation had deteriorated to a dangerous level. On the other hand, if the voltage across the oil film decreased to zero, it would indicate that there was direct contact between the shaft 8 and the bearing pad 12, and the warning light would indicate this condition.

From the above description, it can be seen that a very uncomplicated device has been disclosed which will adequately indicate when the ground insulation is deteriorating. Furthermore, this device will indicate when there is an insufficient oil film between the shaft and the bearing. Additionally, this device is not placed in a position where, if it were to fail, it would provide a direct short between the bearing and ground which would result in damage to the bearing.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows;

1. In combination with a dynamoelectric machine having an electrically grounded rotating shaft with electrical potential generated therein, a bearing rotatably supporting said shaft on a film of oil, said oil providing electrical insulation between said shaft and said bearing sufficient to cause a voltage potential across said oil film and electrical insulation isolating said bearing from electrical ground, a device for detecting electrical potential from said bearing to said ground comprising a voltage sensing means connected across said shaft and said bearing, said voltage sensing means indicating an increase in voltage when there is a condition exposing the unit to an increase in current flow from said bearing to said ground.

2. The combination set forth in claim 1 wherein said voltage sensing means indicates a decrease in voltage when there is a short across the oil film. 

1. In combination with a dynamoelectric machine having an electrically grounded rotating shaft with electrical potential generated therein, a bearing rotatably supporting said shaft on a film of oil, said oil providing electrical insulation between said shaft and said bearing sufficient to cause a voltage potential across said oil film and electrical insulation isolating said bearing from electrical ground, a device for detecting electrical potential from said bearing to said ground comprising a voltage sensing means connected across said shaft and said bearing, said voltage sensing means indicating an increase in voltage when there is a condition exposing the unit to an increase in current flow from said bearing to said ground.
 2. The combination set forth in claim 1 wherein said voltage sensing means indicates a decrease in voltage when there is a short across the oil film. 